Fluid Pulmonary Surfactant Membranes Of Torpid Animals Possess An Orderly Solid-Like Phase At Low Body Temperatures

Pulmonary surfactant (PS), a lipo-protein complex, regulates interfacial surface tension of the lung. The fluctuations in body temperatures of heterothermic mammals correlate with fluctuations in surfactant lipid composition as well as function. Previously we speculated that the higher levels of cholesterol during torpor will reduce phase transition temperature (Tm) enabling PS to remain fluid over a broader range of temperatures. However, these compositional changes do not explain how the surfactant can attain low surface tensions without suffering film collapse at the interface. To gain a better understanding of the molecular interactions that take place at the air-water interface, responsible for surfactant to remain surface active at low temperatures, we explored the thermodynamic interactions and phase-transitional behavior of pulmonary surfactant membranes of heterothermic mammals namely fat-tailed dunnarts (Sminthopsis crassicaudata) and Gould's wattled bats (Chalinolobus gouldii). Thermodynamic studies were conducted with fluorescence spectroscopy by LAURDAN (6-dodecanoyl-2-dimethyl-aminonaphtalene), fluorescence anisotropy by DPH (1, 6-diphenyl-1, 3, 5 hexatriene) and differential scanning calorimetry. We also conducted epifluorescence and atomic force microscopic studies to visualise phase coexistence of surfactant membranes of these animals. Surfactant membranes of torpid animals showed gel-to-fluid transitions at lower Tm and lower enthalpy compared to warm-active animals indicating a more fluid like surfactant. However, at low temperatures, fluorescence spectroscopy and anisotropy studies showed that surfactant from torpid animals possessed a dehydrated solid-like ordered phase similar to that of the warm-active group. This trend was further confirmed by microscopic studies, which revealed structural differences in the morphology and distribution of compression-driven segregated lipid domains in surfactant films. This suggests that in torpid animals, surfactant alters its composition as an adaptation to reduced body temperatures but retains its function by making structural re-arrangements of the surface active film.